Most vehicles today include a variety of safety devices. One such safety device is the airbag. Airbags are soft bags located between a driver or passenger and the vehicle in a moving direction of the vehicle the airbags are stored in a compressed format during normal operation of the vehicle. However, when the vehicle is in a crash, the airbags are rapidly inflated to provide a soft surface to cushion the impact for the driver or passenger.
When a crash occurs, the airbags must be inflated very quickly for them to be of any use. One way to achieve this rapid inflation is through the use of a squib to ignite a rapid chemical reaction that fills the air bag with gas (e.g., nitrogen, argon, or any suitable gas). In such a system, when a crash is detected, an airbag controller provides a fire current to the squib that should be sufficient to fire off the squib. It is therefore necessary to make certain that the airbag controller will provide to this fire current at the appropriate time (i.e., when a crash occurs), and that the fire current will be sufficient to detonate the squib.
It is possible, of course, to test an airbag control circuit when it is manufactured, and before installation in a vehicle. However, this neglects the possibility that the airbag control circuit may be damaged or otherwise altered during installation of the airbag system such that it was no longer capable of providing a sufficient fire current at the appropriate time. It would be more desirable to test the airbag control circuit after the airbag system was installed in the vehicle.
An additional problem with squib testing is called dudding. Dudding occurs when a current is passed through the squib that is not sufficient to detonate it, but that renders the squib unable to detonate in the future, effectively burning it out without detonating it. A typical squib used in an airbag system requires about 1.0-1.2 A to detonate. However, a lower current passed through the squib can cause dudding. A current in the range of 10 mA can safely be passed through the squib without causing dudding. Therefore, it is desirable to keep any current that passes through a squib during testing in a range of about 10 mA or lower.
It would therefore be desirable to provide an airbag control circuit that included self-diagnostic circuitry that would allow it to test itself after installation in a vehicle as to whether it can provide a sufficient fire current when needed to set off a squib in an airbag system. Furthermore, it would be desirable if this airbag test and control system did not ever pass a current through squib that was high enough to cause the squib to dud.